1. Field of the Invention
This invention relates generally to a multimode filter, and more particularly to a multimode filter in an optical storage device.
2. Description of the Related Art
Optical storage devices are widely utilized in many fields. CD-ROMs have become basic devices on personal computers (PCs). With the popularity of CD-Rs and CD-RWs, it is more convenient for users to record data or music. Conventionally, the recording mode of optical disk is constant linear velocity (CLV). As a result, recordable optical devices, for example CD-R, CD-RW, DVD-R, DVD-RW, DVD+RW or DVD-RAM, are recorded with CLV mode. It does not matter whether the position of the pick-up head of the optical device is at the outer or inner side of the optical disk, the speed of recording data into the disk is always kept constant. When the optical storage device operates with high rotational speed, the speed of data recording into the disk is limited by the rotational speed of the spindle motor. Such bottleneck can be resolved by using constant angular velocity (CAV) recording mode. For example, the rotational speed of the inner side of the disk is 8000 rpm when the optical storage device records data with 16× CLV mode. If the recording speed of the optical storage device is changed to 16× CAV mode at the inner side of the disk, the rotational speed at the outer side of the disk becomes 40×.
To label the location of the data in the disk, the tracks of the disk are divided into many sectors units. These sector units include minutes, seconds and blocks. A latest development in labeling is to form the edges of the tracks with a shape like a wave profile. As a result, when the disk is rotated, the reflected laser beam by the edges of the tracks with a shape like a wave profile forms a wobble signal. By using the frequency modulation (FM) the absolute time in pre-groove (ATIP) is encoded in the wobble signal. Thus, when the optical storage device receives the reflected signals from the reflected laser beam, it extracts the wobble signal from the reflected signals and decodes the wobble signal to attain the ATIP, thereby the position of the pick-up head is known.
FIG. 1A is the profile of a recordable disk. Disk 100 is a recordable disk, for example CD-R, CD-RW, DVD-R, DVD-RW, DVD+RW or DVD-RAM. The disk 100 is grooved with tracks 130 for data storage. FIG. 1B shows the detailed structure of tracks 130. As shown in FIG. 1B, the edges 135 of the tracks 130 are formed with a shape like a continuous wave profile. The position of the pick-up head is known by extracting the wobble signal and decoding the ATIP. Thus, extracting the wobble signal is critical for identifying the position of the pick-up head. FIG. 1C shows the relationship of a photo-detector (PD) and the tracks 130. The PD includes a sensor 171, a sensor 173, a sensor 175, and a sensor 177. The spot area 150 of the laser is shown by dotted line. The sensors 171, 173, 175, 177 can sense the laser simultaneously. When the optical storage device is using a CLV mode for recording, the frequency of the wobble signal is 22.05 KHz±1 KHz, where the center frequency is fixed to 22.05 KHz. Since the ATIP is encoded in the wobble signal, the ATIP can be known by extracting the wobble signal. A common way to extract the wobble signal is using the main beam push pull signal, i.e. the tracking error signal. If the values sensed by the sensors 171, 173, 175, 177 are A, B, C, and D respectively, then the main beam push pull signal is (A+D)−(B+C). By filtering the main beam push pull signal with a CLV mode filter of center frequency of 22.05 KHz, a narrow bandwidth wobble signal can be extracted.
Turning now to FIG. 2, it shows a device for extracting a wobble signal when the optical storage device is using CLV mode. By using a CLV mode filer 270 to filter a main beam push pull signal 250, the wobble signal 255 is extracted. Since the center frequency of the CLV mode filter 270 is 22.05 KHz when the speed of the optical storage device is 1×, the frequency of the narrow bandwidth wobble signal 255 extracted from the CLV mode filter 270 is also 22.05 KHz.
When the speed of the optical storage device with CLV mode is 1×, since the frequency of the wobble signal is fixed to 22.05 KHz±1 KHz, the wobble signal can be extracted by a filter with a fixed center frequency. However, when the optical storage device is using CAV mode, the frequency of the wobble signal increases with the linear velocity. The frequency of the wobble signal increases from 22.05 KHz to 22.05×2.505 KHz. In other words, the filter with fixed center frequency is not suitable for CAV mode. Since the frequency of the wobble signal is changed with the radial position of the pick-up head for CAV mode, a method is proposed for adjusting the center frequency of the filter dynamically to extract the wobble signal. However, the method can only be realized when the frequency of the wobble signal is known. The moving speed of the pick-up head is quite high when it is searching data; it is difficult to know the exact frequency of the wobble signal at the moment. Thus, the idea of dynamically adjusting the center frequency of a filter is not realized yet and becomes a bottleneck of developing CAV technology.